Chapter 7:
Biofilm Formation by Cryptococcus neoformans

Affiliations: 1: New York Institute of Technology, College of Osteopathic Medicine, Department of Biomedical Sciences, Old Westbury, NY 11568;
2: Department of Molecular Microbiology and Immunology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD 21205

Historically, microbiologists have studied microbes that cause infectious diseases by analyzing microbial cells grown in suspension (planktonic) in the laboratory. This tradition derives in great part from the early influences of Koch’s postulates, which emphasized working with pure cultures. Unfortunately, this growth in pure cultures has little to do with the growth of microbes in “natural” or host environments. Advances in confocal microscopy and molecular genetics in the last two decades have provided evidence that biofilm formation represents the most common mode of growth of microorganisms in nature. This growth form presumably allows microbial cells to survive in hostile environments, enhances their resistance to physical and chemical pressures, and promotes metabolic cooperation (1). In fact, it is estimated that approximately 80% of all bacteria in the environment exist in biofilm communities, and more than 65% of human microbial infections involve biofilm formation (2). Microbial biofilms are dynamic communities of microorganisms strongly attached to biological and nonbiological substrata that are enclosed in a self-produced protective exopolymeric matrix (EPM) (3).

Model of antibody-mediated inhibition of C. neoformans biofilm formation. In the absence of mAb, C. neoformans cells release capsular polysaccharide which is involved in attachment to the plastic surface. In the presence of a mAb specific to C. neoformans polysaccharide capsule, the immunoglobulin prevents capsular polysaccharide release, which blocks the adhesion of the yeast cells to the surface. Light microscopic images of spots formed by C. neoformans during ELISA spot assay. Images were obtained after 2 h of incubation of fungal cells in the absence and presence of GXM-binding mAb in a polystyrene microtiter plates. Scale bar: 50 μm. The model and light microscopy images in this figure were originally published elsewhere (14). doi:10.1128/microbiolspec.MB-0006-2014.f2

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Figure 2

Model of antibody-mediated inhibition of C. neoformans biofilm formation. In the absence of mAb, C. neoformans cells release capsular polysaccharide which is involved in attachment to the plastic surface. In the presence of a mAb specific to C. neoformans polysaccharide capsule, the immunoglobulin prevents capsular polysaccharide release, which blocks the adhesion of the yeast cells to the surface. Light microscopic images of spots formed by C. neoformans during ELISA spot assay. Images were obtained after 2 h of incubation of fungal cells in the absence and presence of GXM-binding mAb in a polystyrene microtiter plates. Scale bar: 50 μm. The model and light microscopy images in this figure were originally published elsewhere (14). doi:10.1128/microbiolspec.MB-0006-2014.f2

Light microscopy images of the EPM of a mature C. neoformans biofilm stained with GXM-specific mAb. Images of a mature biofilm show that capsular-binding mAb binds and darkly stains shed capsular polysaccharide. (A) Picture was taken using a 10× power field. Scale bar: 50 μm. (B) Picture was taken using a 40× power field. Scale bar: 10 μm. Black and white arrows denote yeast cells and EPM, respectively. These light microscopy images were originally published elsewhere (16). doi:10.1128/microbiolspec.MB-0006-2014.f3

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Figure 3

Light microscopy images of the EPM of a mature C. neoformans biofilm stained with GXM-specific mAb. Images of a mature biofilm show that capsular-binding mAb binds and darkly stains shed capsular polysaccharide. (A) Picture was taken using a 10× power field. Scale bar: 50 μm. (B) Picture was taken using a 40× power field. Scale bar: 10 μm. Black and white arrows denote yeast cells and EPM, respectively. These light microscopy images were originally published elsewhere (16). doi:10.1128/microbiolspec.MB-0006-2014.f3

Schematic of radioimmunotherapy of a biofilm with an antibody labeled with alpha-emitting radionuclide. The “direct hit” effect is the killing of a cell by radiation emanating from a radiolabeled antibody molecule bound to this cell. “Cross-fire” is the killing of a cell by radiation emanating from a radiolabeled antibody bound to an adjacent or a distant cell. “Bystander” denotes the death of an unirradiated cell through the signaling from irradiated cells. doi:10.1128/microbiolspec.MB-0006-2014.f4

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Figure 4

Schematic of radioimmunotherapy of a biofilm with an antibody labeled with alpha-emitting radionuclide. The “direct hit” effect is the killing of a cell by radiation emanating from a radiolabeled antibody molecule bound to this cell. “Cross-fire” is the killing of a cell by radiation emanating from a radiolabeled antibody bound to an adjacent or a distant cell. “Bystander” denotes the death of an unirradiated cell through the signaling from irradiated cells. doi:10.1128/microbiolspec.MB-0006-2014.f4

4.MartinezLR,, CasadevallA.2006. Cryptococcus neoformans cells in biofilms are less susceptible than planktonic cells to antimicrobial molecules produced by the innate immune system. Infect Immun74:6118–6123.[PubMed][CrossRef]